The part most people neglect entirely: being seen from the side.
A front light and a rear light cover you from directly ahead and directly behind. A large share of urban collisions happen at junctions, where a driver is approaching from the side and sees neither.
What helps, cheaply:
Reflective sidewalls on tyres, or spoke reflectors. Genuinely effective and passive: no batteries, always working.
Reflective material low down and moving. Ankles and pedals. Movement is what the eye picks up, and a moving reflector reads as a person on a bike rather than as an ambiguous light.
Something on the sides of your bag or jacket.
Also worth knowing about flashing versus steady: a flashing light is easier to notice and harder to judge distance and speed from. The usual compromise is one steady and one flashing at each end if you have two, or steady on unlit roads where you actually need to see, and flashing in town where being noticed matters more. Some places also have rules requiring a steady light, so worth a check.
And for the rear: a steady rear light plus a reflector is more useful than a very bright flashing one, which some drivers find genuinely disorienting at close range.